As is well known in the gas turbine engine art, it is typical for an aircraft fuel pumping system to utilize one or more centrifugal fuel pumps and, for starting, a supplementary high-pressure positive displacement pump for providing pressure that a properly sized centrifugal pump cannot supply. The problem with this design is that it is relatively bulky, heavy, and mechanically complex. There has been a continuing search in the art for relatively lightweight and versatile arrangements for providing the starting pressurized fuel.
Heretofore, a separate problem in such situations is that the engine itself typically has several control actuation loops and actuators, such as engine nozzle actuators, compressor variable geometry vane actuators, or other critical actuators that are essential to engine operation. Frequently, at least one of these loops and/or actuators employs a hydraulic fluid other than the fuel. Just as fuel pumps can fail, so can hydraulic pumps.
Thus, it is desirable to provide a simple reliable backup capability for the fuel pump and the hydraulic pump that adds little bulk or weight to the base system. Nevertheless, it is necessary to keep the respective fluid circuits for the fuel and the hydraulic fluid isolated from each other.